kswapd consumes 100% CPU when highest zone is small

[Jerry Lee <leisurelysw24@xxxxxxxxx>]

Hi,

I have a x86_64 system with 2G RAM using linux-3.12.x.  During copying large
files (e.g. 100GB), kswapd easily consumes 100% CPU until the file is deleted
or the page cache is dropped.  With setting the min_free_kbytes from 16384 to
65536, the symptom is mitigated but I can't totally get rid of the problem.

After some trial and error, I found that highest zone is always unbalanced with
order-0 page request so that pgdat_blanaced() continuously return false and
kswapd can't sleep.

Here's the watermarks (min_free_kbytes = 65536) in my system:
Node 0, zone      DMA
  pages free     2167
        min      138
        low      172
        high     207
        scanned  0
        spanned  4095
        present  3996
        managed  3974

Node 0, zone    DMA32
  pages free     215375
        min      16226
        low      20282
        high     24339
        scanned  0
        spanned  1044480
        present  490971
        managed  464223

Node 0, zone   Normal
  pages free     7
        min      18
        low      22
        high     27
        scanned  0
        spanned  1536
        present  1536
        managed  523

Besides, when the kswapd crazily spins, the value of the following entries

in vmstat increases quickly even when I stop copying file:

pgalloc_dma 17719
pgalloc_dma32 3262823
slabs_scanned 937728
kswapd_high_wmark_hit_quickly 54333233
pageoutrun 54333235

Is there anything I could do to totally get rid of the problem?

Thanks